An example of this type of turbomachine is shown in FIG. 1 which shows an airplane turbojet 1 of well known type. The turbojet 1 comprises first and second rotary assemblies 10, 9 surrounded by a stator 2, and separated from said stator by a main flow path 3 of annular section. The main flow path 3 is occupied by stages of blades secured alternately to said rotary assemblies 9 and 10 and to the stator 2 so as to accelerate and compress the gas in order to benefit from the energy that it releases while expanding, after combustion of the fuel. Going from the front to the rear, there are to be found: the blades of a low-pressure compressor 4, the blades of a high-pressure compressor 5, a combustion chamber 6, the blades of a high-pressure turbine 7, and the blades of a low-pressure turbine 8. The first rotary assembly 10 comprises the rotor of the low-pressure compressor 4, the rotor of the low-pressure turbine 8, and a first shaft 12 providing the connection between the two above-mentioned rotors, and also referred to as a “low-pressure” shaft or a main shaft. The second rotary assembly 9 comprises the rotor of the high-pressure compressor 5, the rotor of the high-pressure turbine 7, and a second shaft 11 providing the connection between the two above-mentioned rotors 5 and 7, and also referred to as a “high-pressure” shaft. Since the turbojet 1 comprises two rotary assemblies or spools 9 and 10, it is generally referred to as a twin-spool turbojet.
The first and second shafts 12 and 11 are coaxial and rotate at different speeds, the speed of rotation of the first shaft 12 being less than the speed of rotation of the second shaft 11. Both shafts are supported by means of bearings connected to the stator 2. Going from the front to the rear, there are to be found: a front bearing 13 for the first shaft 12, a front bearing 14 for the second shaft 11, a rear bearing 15 for the second shaft 11, and a rear bearing 16 for the first shaft 12. As their active elements, the bearings comprise one or two ball- or roller-bearings that enable the shafts 11 and 12 to rotate at high speed, independently from each other; the shafts 11 and 12 are thus completely separate from each other and mechanically independent. However it should be observed that they are separated by only a small amount of clearance over a rather long proximity zone 17 situated substantially in the vicinity of the front bearing 14 of the second shaft 11.
Modern airplane turbojets have a high compression ratio and a high by-pass ratio. They are thus provided with an auxiliary flow path 18 surrounding the main flow path 3, with air traveling along said auxiliary flow path and being mixed with the combustion gas at the rear of the low-pressure turbine 8 (such turbojets are called turbofans). The air traveling along the auxiliary flow path 18 is accelerated by the blades of a fan 19 that is secured to the first rotary assembly 10 and that extends in front of the low-pressure compressor 4. The blades of the fan 19 have a very large diameter and considerable inertia. They are also subject to rupture when the airplane is in flight and when a foreign body, such as a bird, accidentally comes into contact with said blades.
As soon as a fan blade is ruptured, a significant unbalance occurs in the first rotary assembly 10, thereby producing significant vibration forces thereon, which forces are transmitted to the second rotary assembly 9 and to the stator 2 via the front bearing 13. The damage resulting from such excessive forces is capable of propagating throughout the turbojet 1. For this reason, it is known to use a “fusible” front bearing 13, i.e. a bearing that is capable of being broken or of giving way in some other way when an unbalance occurs in the first rotary assembly 10. That type of bearing 13 generally includes a break starter in the vicinity of the first shaft 12, which starter is generally a thin portion connecting it to the stator 2, or small-diameter connection bolts having threaded shanks that may be notched; an example of that type of bearing is described in U.S. Pat. No. 5,417,501. The break starter is designed so as to tear or to rupture when the unbalance occurs, so that the front bearing 13 becomes detached from the stator 2 and ceases to support the first shaft 12, which then becomes free to oscillate by tilting about the rear bearing 16, thereby no longer imparting excessive force on the stator 2. Faced with such a problem, the pilot generally shuts down the corresponding turbojet (i.e. stops fuel combustion), thereby no longer driving the shafts 11 and 12 in rotation. The speed of rotation of the shafts 11 and 12 thus decreases and the second shaft 11 progressively stops turning. As the airplane continues its flight, the fan 19 driven by the air which passes therethrough continues to turn slowly (relative to its normal speed) and drives the first shaft 12 in rotation; the first rotary assembly 10 and the shaft 12 are said to be windmilling.
Considerable damage may occur from the moment when the front bearing 13 gives way to the moment when the first shaft 12 windmills slowly, i.e. while the shafts 11 and 12 are still turning at high speed and touching each other in the proximity zone 17, as shown in FIG. 2, due to the fact that the first shaft 12 is rocking around the rear bearing 16. When the shafts come into contact with each other, considerable heating is caused by the friction resulting from the speeds of rotation of said two shafts 11 and 12 that are very different and very high (e.g. 4,500 revolutions per minute (rpm) and 17,000 rpm. Heat dissipation is concentrated in a limited contact zone 20 around the circumference of the first shaft 12, and is such that the shaft 12 is damaged at that location and passes to a metallurgical state where it is weaker, and even likely to rupture. There is then a risk of losing the fan 19 when the first shaft 12 ruptures. Moreover, since contact between the shafts also damages the second shaft, both shafts 11 and 12 need to be replaced during repair work.
To avoid such drawbacks, a known solution, described in French patent No. 2 773 586 attempts to ensure that the two shafts 11 and 12 quickly start functioning at the same speed so that friction, and hence heating, is limited. That solution consists in covering the proximity zone 17 of the first shaft 12, i.e. the shaft rotating more slowly than the second shaft 11, in a covering having low thermal conductivity and capable of being machined by milling or the equivalent. Such a covering consists of a layer of zirconia, yttrium-containing zirconia, alumina, boride, or carbide.
Nevertheless, despite the low thermal conductivity of the covering, it is observed that the capacity of said covering for machining by milling leads to a considerable rise in temperature (often in excess of 1000° C.) which has a negative effect on the properties of the two shafts 11 and 12, thereby not sufficiently removing the risk of the first shaft 12 rupturing, and the dramatic consequences that such a rupture would incur.